on a 2004 Toyota Corolla with 182,00 miles…when i took the car to the shop about 2 months ago and the car had code p0133, p0420 and p0171..the mechanic tells me if i install a new oxygen sensor the codes will probably clear. so i had a new o2 sensor installed..3 days later i get just the p0420.. since its expensive to install a new catalytic converter I Drive the car like that for a month , try to save money..about 5 days ago i notice now i have the p0133 and the p0420 again, I Take it back to the shop since the oxygen sensor is under warranty. The mechanic tells me its best to install a catalytic converter. that the codes might clear, but i always thought u have to clear the p0133 b4 installing the sensor..is he giving me the run-around?

RESPONSE

The codes do not have to be cleared before installing a sensor. Generally the codes are cleared after the sensor or parts are installed and tested. You stated above that the code p0133 had gone 3 days after the O2 Sensor was replaced. So the code was gone(cleared). The Code P0420 usually means you need to replace the catalytic converter. The converter does absolutely nothing for the drivability of the vehicle. This is for Government mandated emissions only. You do not have to replace the converter but the check engine light will remain on for the code p0420 until you do.

When the P0171 is combined with the P0174 code, it’s very likely that the problem is caused by an intake leak. If there are no intake leaks, the next step is to replace the air filter and clean the Mass Air Flow Sensor. If the problem persist the front oxygen (O2) sensor may need to be replaced. Also replacing the converter will not fix the code p0133. The mechanic said it MIGHT which leaves you open for IT MIGHT NOT. Therefore I would say NO to the converter until the P0133 code is taking care of unless you want to of course.

I think your mechanic is trying to help you clear the codes but I also think your mechanic may be overlooking a possible intake air leak. Or there may be something more involved. There may be a damaged wire in the O2 Sensor circuit itself. And it is possible the o2 sensor is bad, but it can be checked with a scan tool. I would assume the o2 Sensor operation was checked since you took it back in for warranty purposes.

Code Information

The P0420 code means that the vehicle’s control module has detected that the three-way catalytic converter is not working properly (is not as efficient as the factory is expecting). Replacing the oxygen (O2) sensors may sometimes fix the code, but in most cases the catalytic convert needs to be replaced to fix the problem. If the P0420 code is combined with other codes, try fixing the other codes first.

The front heated oxygen sensor (or O2 sensor 1) is placed into the exhaust manifold. It detects the amount of oxygen in the exhaust gas compared to the outside air. The heated oxygen sensor 1 has a closed-end tube made of ceramic zirconia. The zirconia generates voltage from approximately 1V in richer conditions to 0V in leaner conditions. A heated oxygen sensor 1 signal is sent to the Engine Control Module (ECM). The ECM adjusts the injection pulse duration to achieve the ideal air-fuel ratio. The ideal air-fuel ratio occurs near the radical change from 1V to 0V.